GRUB, which you might or might not know, so I'll take the chance of sounding silly here; is the default boot-loader for RedHat these days. The problem you're having now is that one boot loader is trying to summon another, which doesn't exactly ever work. See if you can get into RH with a rescue disk and then scrape GRUB off there. I don't exaclty know how to do that last step because I use Lilo on everything.
Hope that's a start.